This invention relates to water-soluble core materials for use in investment casting processes, and more particularly to improved methods for inspection of the completeness of removal of water-soluble cores in the primary wax pattern used to produce investment castings.
Compositions for the construction of disposable patterns used in investment casting processes, also called lost wax processes, are selected for several characteristics, including such important properties as dimensional reproducibility and the ability to produce a highly accurate surface finish in the molded disposable pattern. Because such properties are critical to many products manufactured by lost wax processes, repeated efforts have been and are being made to improve such properties of pattern-forming compositions.
The quality and properties of an investment casting depend on the quality of the disposable pattern, which, in turn, depends on the characteristics of the pattern-forming compositions of which the disposable patterns are molded. Disposable thermoplastic patterns are usually formed by heating and melting a wax-like thermoplastic composition and injecting the molten composition under pressure into a mold, and then cooling the composition until it solidifies to form a disposable pattern. The disposable pattern then is removed from the mold, assembled with the other patterns if necessary, and then encased in a mold forming material, usually a ceramic material, in accordance with one of a variety of known methods, thereby forming a shell or cast about the disposable pattern.
Next, the disposable pattern is removed by melting or vaporizing the pattern material at a moderately elevated temperature by autoclaving. Substantially all of the remainder of the pattern material is removed at a substantially higher temperature by vaporization or burning or both so that, except for any ash residue from the pattern material, the inner surface of the shell or mold is clean. The shell or mold is then ready for one-time use for forming an investment cast part. A text describing known procedures used in lost wax processes is entitled Investment Casting, H. T. Bidwell, Machinery Publishing Co., Ltd., England, 1969.
However, many times the part to be cast is of a shape that does not lend itself to such simple method. Frequently, disposable wax patterns with simple or complicated internal configurations, undercuts, holes or grooves are required. Simple internal configurations in disposable wax patterns often may be produced with sliding pins or metal pullout cores because the disposable wax pattern offers no resistance to the extraction of such pins or cores.
By contrast, many disposable patterns containing intricate internal configurations often cannot be produced by means of such relatively simple techniques. In such cases, a "core" of shape corresponding to the internal configuration of the pattern is produced from a moldable water-soluble, wax-like composition, especially a thermoplastic composition, often containing filler and other ingredients. For example, the core composition may comprise polyethylene glycol, especially of molecular weight of at least 1,000 and as high as 6,000 or more, a filler such as mica, rayon or glass fibers for strength, and an ingredient like powdered sodium bicarbonate for facilitating dissolution and break-up of the composition upon exposure to water.
After the water-soluble core is molded and solidified, it is placed into a mold having a cavity conforming in shape to the exterior configuration of the part to be produced, and a molten pattern material is injected into the mold and solidified around the core to form a pattern of solidified pattern material around the core within the mold. The pattern then is exposed to water such as by immersion into an aqueous solution which decomposes the core by dissolution and the resulting breaking up of the core, thereby leaving a pattern having an opening therein of the desired shape.
The resulting heat disposable wax pattern with a void or voids of desired shape is then encased in a ceramic shell. It is critical that all of the water soluble core material be dissolved and removed from the pattern material prior to such encasing. If the water soluble core is not completely from the core, the residue causes inaccuracy in the finished casting and further leads to a reaction between the inorganic residue from the core material and the molten metal as the molten metal enters the mold. The reaction results in a scrap casting.
Accordingly, it is important to be able, before encasing the pattern material in ceramic, to inspect the cavity left from the dissolution of the core material to ensure that the cavity is free of residue from the core material. Such inspection is difficult because of the inaccessibility of view of areas of the cavity. Because of this difficulty, X-rays have been employed for inspection, but such techniques are expensive and involve the dangers inherent in the use of X-rays. Accordingly, simpler, safer and less expensive techniques for such inspection are desired.